Spatial heterodyne spectroscopy (SHS) was conceived in the late 1980s by Prof. Fred Roesler and his graduate student John Harlander at the University of Wisconsin. SHS interferometers include a beamsplitter two gratings, and two (optional) field-widening prisms. A detailed description of SHS can be found in “Robust monolithic ultraviolet interferometer for the SHIMMER instrument on STPSat-1”, APPLIED OPTICS, vol. 42, Nol. 15, 20 May 2003, by J. M. Harlander et al. The optical components of SHS interferometers have to be mounted within tight. interferometeric tolerances to achieve high performance. There are generally two types of conventional SHS interferometer configurations, those with discrete optical elements and those with a monolithic optical element.
FIG. 1 illustrates an exemplary convention SHS spectrometer having an interferometer of discrete optical elements. In the figure, the spectrometer 100 includes input optics, an interferometer and output optics. The input optics include an input aperture 102, and collimating lens 104. The interferometer includes a beam splitter 106, prism 108, prism 110, grating 112, and grating 114. The output optics include focusing lens 116, collimating lens 118 and detector 120.
In operation, input light passes through input aperture 102 and diverges to collimating lens 104. Collimated light λ1 includes an incident wave front 122. Collimated light λ1 is then incident upon beam splitter 106. A first portion of collimated light λ2 is reflected toward prism 108, which is then refracted toward grating 112 having a Littrow angle 124. Grating 112 reflects light λ3 back though prism 108 and toward beam splitter 106, where light λ3 is partially reflected toward lens 104 and partially transmitted toward lens 116. The output optics portion is designed to image the grating planes 112 and 114 onto the detector 120. Here, the partially transmitted light λ6 includes a wave front 128 and is focused by lens 116 to a point 134. The light λ6 then diverges toward lens 118 to be imaged on detector 120. A second portion λ4 of collimated light λ1 is transmitted through beam splitter 106 toward prism 110, which is then refracted toward grating 114 having a Littrow angle 126. Grating 114 reflects light λ5 back through prism 110 and toward beam splitter 106, where light λ1 is partially transmitted toward lens 104 and partially reflected toward lens 116. In the output optics portion, the partially reflected light λ7 includes a wave front 130 and is focused by lens 116 to a point 134. The light λ7 then diverges toward lens 118 to be imaged on detector 120.
Wave front 128 constructively and destructively interferes with wave front 130, such that that image detected by detector 120 is an interference pattern. An example of such an interference pattern is illustrated in FIG. 3. The characteristics of the pattern are based on the wavelength of the light λ1 and the angle 132 between wave front 128 and wave front 130. Angle 132 is mainly based on the frequency of the input light λ1 and the structure and angle of gratings 112 and 114. Field-widening prisms 110 and 108 are optional, and merely compensate for non-paraxial rays within the interferometer, in order to increase the throughput.
The optical components of interferometer 100 are individually mounted using commercial or custom-made mechanical mounting techniques like lens holders and three-point mounts. Due to the tight tolerances, the holders are typically adjustable, so the interferometer can be aligned after its assembly. In order to build a rugged interferometer, the holding fixtures have to be very stiff and the adjustable optics mounts get complicated. For example, all elements may typically be held individually in adjustable mounts within a steel fixture. The weight of such an interferometer (−7 kg) is dominated by the steel fixture that is necessary to keep the optical components in position. Alternatively a laboratory breadboard interferometer assembly may use commercial fixtures to hold the individual components. This set up is particularly sensitive to vibration since the commercial mounts are not optimized for stiffness. In any event such conventional discrete optical element types of SHS interferometers generally are relatively heavy as a result of the required mounting systems and have very time consuming adjustment procedures.
The conventional discrete optical elements design of SHS is appropriate for laboratory investigations but the inherent lack of ruggedness excludes these designs from virtually all operational applications that are based on platforms like land vehicles, airplanes, unmanned aerial vehicles (UAVs), satellites, or even a handheld device which has to withstand rough environments to be reliable. Moreover, the realignment of a misaligned interferometer is not trivial and requires a trained person and appropriate equipment, thus further hampering the use of this assembly technique for commercial or military devices.
FIG. 2 illustrates a conventional SHS interferometer comprising a monolithic design. FIG. 2 does not include input optics, output optics or a detector as illustrated in FIG. 1. However, one of skill in the art would understand the operation of the configuration illustrated in FIG. 2 Within a spectrometer.
In the figure, an interferometer 200 includes a first leg portion 202, a second leg portion 204 and an optical beam splitter 206 having a half mirror 208 therein. First leg portion 202 includes a reflective grating 210, a spacer 212, a prism 216 and a spacer 214. Similarly, second leg portion 204 includes a reflective grating 218, a spacer 220, a prism 224 and a spacer 222.
In operation, input light 226 passes into beamsplitter 206 and a portion of which, 240, ultimately exits. Specifically, input light 226 is incident upon half mirror 208 and first portion 228 of input light 226 is reflected toward first leg portion 202 and a second portion 232 is transmitted toward second leg portion 204. In a manner similar to the system illustrated in FIG. 1, portion 228 of the input light transmits through prism 216, which is then refracted by an angle toward grating 210. Grating 210 reflects the light back through prism 216 and toward beam splitter 206, where the light is partially reflected at half mirror 208, wherein portion 236 is transmitted to an output face of beamsplitter 206 and wherein portion 240 is reflected to the input face of beamsplitter 206. Similarly, portion 232 of the input light transmits through prism 224, which is then refracted by an angle toward grating 218. Grating 218 reflects the light back through prism 224 and toward beam splitter 206, where the light is partially reflected at half mirror 208, wherein portion 238 is reflected to an output face of beamsplitter 206 and wherein portion 242 is transmitted to the input face of beamsplitter 206. Output 244 is a combination of light portion 236 and light portion 238, which eventually is detected as an interference pattern.
The main difference between the system illustrated in FIG. 1 and the device illustrated in FIG. 2 is that interferometer 200 of FIG. 2 includes spacers 212, 214, 220 and 222, which enables interferometer 200 to be monolithic.
The main driver for a monolithic SHS interferometer is its inherent ruggedness which was lacking in the system of FIG. 1. Spacers 212, 214, 220 and 222 maintain alignment of the remaining optical components. In such a conventional monolithic type device, the optical components are optically contacted with spacers to form a truly monolithic piece of glass. Specifically, spacer 214 is in optical contact with beamsplitter 206 and prism 216, spacer 212 is in optical contact with prism 216 and grating 210, spacer 222 is in optical contact with beamsplitter 206 and prism 224, and spacer 220 is in optical contact with prism 224 and grating 218.
Optical contacting is a method where the interfacing surfaces of two components are polished to extremely high flatness (several nanometers) before they are contacted. The close proximity of the flat surfaces causes the van der Waals forces to form a strong bond between the components without any adhesives. The lack of a layer of adhesive between components is beneficial, mainly because the thickness of the adhesive layers does not have to be controlled during assembly, which simplifies the “self alignment” during the interferometer assembly, which is then provided by the spacers alone. In order to get a strong bond and to avoid stress due to unequal thermal expansion coefficients, the spacers are made from the same material as the optical components.
Unfortunately, as a consequence of the strong bond between the surfaces, a monolithic interferometer cannot easily be disassembled without risking the destruction of the entire interferometer. The monolithic design provides lighter intrinsically aligned interferometers that are insensitive to vibration. However, polishing the interface surfaces to meet the precision and accuracy required for the optical contacting is very labor intensive, since it has to be performed partly by hand. Therefore, this production technique is very expensive and time consuming (over one hundred thousand USD per interferometer). A monolithic interferometer is appropriate e.g. for one of a kind satellite instruments, but again, it is not suitable for wide spread applications in the commercial or military sector due to its high cost.
In summary, the conventional design and assembly techniques for SHS interferometers seriously impede the development of SHS spectrometers for wide spread applications in the commercial or military sector. The main reasons are either the sensitivity to vibration or the prohibitive cost per unit.
What are needed are SHS interferometers that have the ruggedness of a monolithic device, while significantly reducing the cost per unit. These will finally allow the development of commercial and military SHS devices for application areas where SHS is superior to current spectroscopic techniques like Fourier transform spectroscopy or grating spectroscopy.
There is an increased interest in spectroscopy, such as passive remote sensing for multiple purposes like intelligence gathering (monitoring of exhaust fumes), tactical battlefield applications (chemical threat identification), or tagging, tracking and locating.